The present invention relates to a compression system of a motion image or a still image, and particularly to an image compression system for compressing an image by fixing the amount of compressed bits with respect to a block of a predetermined size (for instance, 8.times.8 or 16.times.16).
Along with the development of computer and digital communication technology, digitization of image information is becoming increasingly necessary for the storage and transmission thereof. However, the. digitizing of image information is disadvantageous in that a massive amount of data is generated. Image information compression techniques are developed to help counteract this disadvantage.
Compression coding of an original image can be classified into motion image coding and still image coding, according to which form of construction is implemented. Motion image coding compresses data by eliminating the redundancy of temporally successive images in the interframe pictures. Meanwhile, still image coding compresses data by eliminating the redundancy in the intraframe. Therefore, since real-time information processing is made in motion image coding, here, the method used for information transmission has a very high speed as well as transmission effectiveness, but image quality is degraded. On the other hand, still image coding has the advantage of providing high quality images although it is difficult to process the information in real time. Due to these factors, these two coding techniques have been developed to supplement each other while maintaining the advantages of each.
The Joint Photographic Experts Group (JPEG) has provisionally decided on a standardized coding system in which a discrete cosine transformation (DCT) coding system is adopted. Here, DCT coding system is the transformation system wherein data is subjected to a two-dimensional DCT, so that a statistical characteristic with respect to the obtained transformation coefficient is obtained, and a quantization method suitable for the statistical characteristic is applied, thereby compressing data. The DCT coding system has almost the same performance as Karhunen-Loeve Transformation, which is the optimum transformation technique with respect to a first order Markov signal source, and thus is frequently utilized for compressing data, filtering, and extracting characteristics.
Since the late 1970's, the endeavor to carry out coding using the statistical characteristics of the DCT coefficients has been made and resulted in the development of the following technique.
That is, an effective coding system using discrete cosine transformation, presented by W. H. Chen and C. H. Smith, is carried out such that an AC energy is segmented into four classes according to activity levels. Here, fewer bits are allocated to a class with less activity, while more bits are allocated to a class with relatively great activity (pp. 1285-1292 of IEEE Trans. Comm., Vol. COM-25 published in 1977 entitled "Adaptive Coding of Monochrome and Color Images"). However, since all classes are segmented to have an equal number of blocks regardless of the simplicity or complexity of an image, this system has problems in that the inherent activity of a block is not considered, and bits are allocated by the relative complexity of the overall image.
In addition, W. H. Chen and W. K. Pratt have suggested a system wherein a scale factor is repeatedly adjusted to maintain a constant amount of coded bits using a rate buffer, thereby enabling an NTSC color image to be transmitted at 1.5 Mbps (pp. 225-232 of IEEE Trans. Comm., Vol. COM-32 published in 1984 entitled "Scene-Adaptive Coder"). The construction of this system is relatively simple. However, since the amount of bits is adjusted while repeatedly changing the scale factor, the characteristic of the block does not correspond to the scale factor in this system. A further problem is that the excessive bits of a current block are transferred to another block. Also, the amount of bits is not previously estimated.
K. N. Ngan, K. S. Leong, and H. Singh have proposed a system which is similar to that of the above Chen and Pratt system in construction, and adopts the human visual system (suggested by Nill on pages 551-557 of IEEE Trans. Comm., Vol. COM-33 published in 1985, entitled "A Visual Model Weighted Cosine Transform for Image Compression and Quality Assessment") to transformation coefficients. In this system, since the scale factors are repeatedly varied in accordance with the distortion between each block and the state of the rate buffer, it has the problem of the block characteristics not corresponding to the scale factors.
In the above-described systems, since the amount of bits is stabilized by varying scale factors according to the coded state of each block, bits are not properly allocated when the scale factors greatly fluctuate. Moreover, a problem in the current block is transferred to the next block, so that the distortion in one block affects a neighboring block, which causes the original image to be impaired unevenly.